Field of the Invention
The field of the invention is that of digital mobile radio systems. The invention applies in particular, although not exclusively, to cellular systems to the GSM 900 standard (Global System for Mobile Communications in the 900 MHz band), the DCS 1800 standard (Digital Cellular System, 1800 MHz) or the PCS standard (Personal Communication System).
To be more precise, the invention concerns call handover between two relay stations of a cell of a digital cellular mobile radio system.
Conventionally, a cell is associated with a base transceiver station. The base transceiver station uses a signaling carrier specific to it and which supports a broadcast control channel (BCCH). A mobile station in the cell communicates with the system via the base transceiver station. This communication uses a traffic carrier allocated by the base transceiver station. The information corresponding to this allocation is transmitted on the signaling carrier.
Conventionally, each carrier is time-shared, using the time-division multiple access (TDMA) technique. This divides the time axis into successive frames of predetermined duration. Each frame is in turn divided into a particular number of time slots. The recurrence of a particular time slot in each frame corresponds to a physical channel onto which a number of logical channels can be multiplexed.
A mobile station communicates via a traffic channel (TCH), different from those allocated to other mobile stations in the same cell, and receives information concerning the system via at least one signaling channel associated with that traffic channel.
The invention applies to cells in which the base transceiver station uses a plurality of relay stations geographically distributed within the cell. Each relay station includes an antenna. Each antenna is associated with at least one transmit/receive device (or TRX, to use the GSM terminology). Each transmit/receive device includes a downlink traffic channel carrier transmitter and an uplink traffic channel carrier receiver.
The relay stations, and therefore the antennas, are geographically distributed within the associated cell to provide improved coverage and/or to handle a greater density of calls. The various relay stations are associated with a common signaling carrier. To this end, at least some of the relay stations include a downlink signaling channel carrier transmitter and an uplink signaling channel carrier receiver.
All the relay stations can be connected to the next higher level in the hierarchy (which is a base station controller (BSC) in the GSM) via a concentration center. In one implementation, the BSC provides the concentration functions directly and in this case the relay stations are connected directly to the BSC.
The problem arises of handing over calls between two relay stations of the cell. "Conventional" handover is operative between the base stations of two different cells. This "conventional" process is as follows: each base transceiver station transmits its own signaling carrier. The mobile station measures the signaling carriers of adjoining cells continuously. The results of such measurements are regularly transmitted via the current base transceiver station to the base station controller, which decides whether there is an opportunity for handover. When a call must be handed over, the controller decides the target base transceiver station and allocates a target traffic channel therein to the mobile station. This allocation is transmitted to the mobile station so that it can switch from the current traffic channel to the target traffic channel.
Consideration has been given to transposing this "conventional" handover between base stations to handover between relay stations of the same base station (which is no longer "conventional"), by simply substituting the relay station for the base station.
Unfortunately, such transposition is not easy, even impossible. In other words, the "conventional" handover process cannot be easily implemented with the distributed base transceiver station configuration (i.e. one comprising a plurality of relay stations) to which the present invention applies.
Each relay station does not transmit a signal that is specific to it (and therefore enables it to be identified). Consequently, the mobile station is not in a position to communicate with the measurement system enabling the latter to decide whether there is an opportunity for handover, or, for even stronger reasons, to determine the best target relay station to which the call should be handed over.
One objective of the invention is to overcome this major drawback of the prior art.
To be more precise, one objective of the present invention is to provide a method of handing over calls between two relay stations of the same cell of a mobile radio system that is simple and inexpensive.
Another objective of the invention is to provide a method of the above kind which optimizes the use of resources.
Another objective is to provide a method of the above kind that does not require any additional hardware to be developed, either for the mobile stations or for the system.
Another objective is to provide a method of the above kind which limits the number of handovers between relay stations required to obtain a predetermined reception quality.